And for some reason, it reminds me of some of the words to Woodie Guthrie’s classic American folksong that’s about private property, among many other things –

“As I went walking I saw a sign there
And on the sign it said “No Trespassing.”
But on the other side it didn’t say nothing,
That side was made for you and me.
This land is your land, this land is my land
From California to the New York island
From the Redwood forest to the Gulf Stream waters
This land was made for you and me.”

Perhaps you are like me: you don’t like these signs.
?
That’s why Guthrie came to your mind.

Because usually it’s obvious when a spot is private propriety. I don’t need a sign to tell me. And people who don’t respect privacy and private propriety would simply laugh at the sign.
In the end the sign is meaningless.

You’re right, Martina: I don’t really like the sign. It’s irritating – because as you say, it should be obvious when something is “private property” – but the fact that someone has put it up indicates more about their own state of mind (fearful, mistrusting, etc) than anything else.

My late aunt, who was a distinguished historian and researcher, used to give extemporaneous speeches (or rants) about her theory that the invention of ‘private property’ (i.e. when men first put up fences to protect and separate ‘their’ land from outsiders) was the ‘original sin’.